1. Field of the Invention
The present invention relates to micro wiring boards used in semiconductor devices or semiconductor integrated circuits and to methods for making the micro wiring boards. In particular, it relates to a micro wiring board made by a transfer printing technique and to a method for making the micro wiring board.
2. Description of the Related Art
A transfer imprinting technique, unlike its original embossing nanoimprinting technique, is an additive process that does not require a process of etching the resist after transfer.
U.S. Pat. No. 5,772,905 teaches a transfer printing method in which gold is vapor-deposited on a polydimethylsiloxane (PDMS) mold having a protruding feature and activated by oxygen plasma treatment, and then the mold is pressed against a substrate to transfer gold. The patent document also discloses formation of a semiconductor circuit by bonding a plurality of substrates each preliminarily provided with a pattern thereon. This semiconductor circuit operates as an inverter.
In “Metal Transfer Printing and Its Application in Organic Field-Effect Transistor Fabrication” by Z. Wang, J. Yuan, J. Zhang, R. Xing, D. Yan, and Y. Han, Advanced Materials, 15, 1009 (2003), a transfer method in which gold is deposited on a mold composed of PDMS and then the mold with gold is pressed against a polymer-coated silicon substrate to transfer gold is disclosed. The transfer is carried out by heating the substrate at a temperature not less than the glass transition temperature of the polymer so that the adhesiveness of the polymer is increased to facilitate transfer. This document also discloses an example in which wirings are stacked on the same substrate. That is, after transferring a gold line pattern (first layer) having a line width of 50 μm on a substrate, another gold line pattern (second layer) having a line width of 50 μm is formed on the first layer gold line pattern so that the pattern of the first layer and the pattern of the second layer are orthogonal to each other. The document also discloses that the gold line pattern of the second layer is adequately bonded and supported because the area of contact between the gold line pattern of the second layer and the polymer portion outside the gold line pattern of the first layer is sufficiently large.
In “Metal printing with modified polymer bonding lithography” written by Xinhong Yu, Shunyang Yu, Zhe Wang, Dongge Ma, and Yanchum Han, Applied Physics Letters, 88, 263517 (2006), a process of transferring a metal pattern onto a polymer is disclosed. In this process, a PDMS mold is used and transfer is carried out by utilizing the lowering of the glass transition point of the polymer exposed to the solvent vapor.
As described above, micron- to submicron-order electrode wirings have been formed by the transfer printing technique.
However, the transfer printing technique has the following drawbacks in fabricating large-scale circuits. In general, most adhesive layers used for transfer have insulating properties. Thus, in forming interconnections between pads necessary for circuit wiring and metal-metal junctions necessary for vias, conduction between the first metal pattern and the second metal pattern cannot be ensured because the second metal pattern is transferred on the insulating adhesive layer on the first metal pattern. Even when a conductive adhesive is used, it is difficult to form an adhesive layer at a specified position on a nano-level due to limited alignment accuracy.
As disclosed in Advanced Materials, 15, 1009 (2003), in the case where a polymer on a substrate surface outside the first metal pattern region is used as an adhesive layer, the contact area between the overlying metal pattern and the polymer is small. Thus, in the case where a transistor having a line width or an interline space smaller than about 50 μm is to be fabricated to satisfy the needs for higher density wiring or a shorter transistor channel length, it is difficult to stably fix the overlying metal pattern. Moreover, since the adhesive layer must be provided in a region outside the wiring region, it is difficult to increase the wiring density. Furthermore, in the case where patterns are bonded in the height direction only, such as when a via or a laminate is to be formed, the patterns in the second and higher layers lose contact with the adhesive portion. Thus, the patterns cannot be bonded.
Another problem arises when the transfer printing technique is combined with another drawing process such as ink jet printing or screen printing. The problem is wetting of ink on metal patterns. The wettability of the wiring to the ink decreases further if the releasing agent applied on the mold in advance remains on the wiring after transfer. The remaining releasing agent may be removed by an appropriate process such as UV-ozone processing, but damage inflicted on the organic substrate by this process is considerable. Moreover, the film thickness of the wiring formed by ink jet printing or screen printing is large, which renders it difficult to apply the transfer printing technique.